Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
Saleshandy - Saleshandy is a cold email outreach platform that enables marketing and sales teams to scale their email outreach operations seamlessly.
FollowUpThen - The simplest way to schedule an email reminder
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Mailtrack.io - The best email tracking solution I've found. Free
Right Inbox - Schedule Emails in Gmail To Be Sent Later